REVISION: It has been brought to my attention by several commenters that there was a Ragnarok arc in the Thor comics that I was unaware of. During this arc, Loki teamed up with the fire demon Surtur to created weapons forged like his hammer, Mjolnir, which were giving to Loki's enemies. I apologize for not knowing about this arc before. Now I think that this is the most likely plot line for Thor: Ragnarok. However, I still stand by the ideas I had below, albeit most of them being far less likely than before.
Let me start with IMDb's plot synopsis of Thor: Ragnarok:
Thor must confront the gods to the gods when Asgard is threatened with Ragnarok, the Norse apocalypse.
At first glance, this seems like it might be a straightforward representation of the Norse legend with a Marvel twist. This, however, is quite unlikely, for several reasons. The first and foremost of these reasons is this guy:
In the original myths, Loki was half-troll rather than being a full-blooded frost giant. Before Ragnarok, Loki had also been imprisoned by Odin deep underground, beneath the fangs of the World Serpent, a snake so long it wrapped around the roots of the World Tree. The Serpent, Jormungandr, also happened to be Loki's son, as was the wolf, Fenrir, who swallowed Odin. The third and final child of Loki was the goddess Hel, a half-living, half-dead being who ruled the realm of the dead, which was also named Hel.
Now, there is a while to go until Thor: Ragnarok is released, but given that Loki is Marvel Studios' most popular villain, I think they would have mentioned or referenced his children by now, especially considering that they could play such a huge role in the MCU. Also, since Loki is a frost giant and not a half troll, I think that it's highly unlikely he could have such varied children. The closest they could come would probably be a half-frost giant, half-god warrior.
Also, I was unable to find any reference to an event in the comics that was close to the original Ragnarok. Not to mention it seems like bad writing to kill Thor off so close to the Infinity War (I did mention that most of the gods died at Ragnarok, right?). And if Thor does survive, it still wouldn't be close to the original.
No, the likelihood of a highly accurate portrayal of Ragnarok is low. Below are, in my opinion, the most likely choices, beginning with those most likely to be...
The Gods to the Gods
The Celestials, 2,000-foot tall beings who came to Earth and preformed genetic experiments on human that would later result in mutants, have had a previous run-in with the Asgardians. The Destroyer, the suit of fire-blasting armor Thor destroyed in the first movie, was created on Odin's order because he did not believe that the Celestials would honor their promise not to meddle in the affairs of the humans and he wanted a weapon capable of stopping them.
It's possible that the Celestials could arrive on Asgard and end up fighting the gods. Something worth noting is that the Celestials have almost certainly appeared in the MCU, and were tied to the Infinity Stones:
Now, compare that to this:
For those of you who don't recognize the first image, it came from the flashback scene in Guardians of the Galaxy when the Collector was telling Star-Lord and the others about how ancient beings would use the Infinity Stones to lay waste to civilizations and entire worlds.
Of course, that scene could have just been an Easter Egg Marvel used to reference the Celestials and had nothing to do with the MCU, but it could also have something to do with an upcoming plot. If an army of the Celestials does come to Asgard (possibly toting another Infinity Stone?), it could certainly be considered an apocalypse, not to mention totally awesome.
Next Up, a Destroyer Army
This is so, so unlikely, but would be pretty cool. With Loki as King of Asgard, he may order the Asgardians to create another Destroyer, or an army of them, to stop Thor from returning to Asgard. Then, somehow, the Destroyers get set against Asgard.
It's also possible that Loki might create Destroyers that, like in the comics, have to absorb the soul of somebody to animate itself. They could even go along with the plot line where Loki tricked Thor into animating the Destroyer, only to have Thor control the armor with his willpower. This is probably the least likely of my theories.
And Now, Ragnarok
That's Ragnarok, as in the cyborg-Thor clone Tony Stark and Reed Richards created during Marvel's Civil War to help control the anti-registration heroes. I think that if Ragnarok does show up, his origin story will change slightly due to the upcoming Avengers: Age of Ultron. Namely the fact that Stark awakens Ultron, means that it's unlikely that he'll create another semi-mechanical villain for Phase 3.
Also, Ragnarok was created because the original Thor was nowhere to be found during the Civil War. Now, Thor will probably have gone back to Asgard by the time the Civil War begins, and I say that simply because it would keep the movie more faithful to the comics. Of course, this doesn't mean that Thor won't be in the movie, it's just highly probable.
But just because Stark probably won't create Ragnarok doesn't mean that he won't appear. Perhaps Loki manages to create some sort of a magical clone. One thing that people have been quick to notice about the Avengers: Age of Ultron trailer is that Thor seems to be lacking his hammer quite a bit. Some people have said that maybe, as King of Asgard, Loki manages to make Thor 'unworthy' to wield the hammer. Or, in my opinion, he simply removes Odin's spell that only one worthy to wield the power of Thor may wield Mjolnir and manages to take the hammer. Rather than wielding it himself, Loki creates a clone of Thor (possibly, if he creates the Thor-clone, he may not have to remove the spell), or possibly a Destroyer-type thing to use Mjolnir. In fact, if Loki figures out how to make a clone of Thor, I don't see why he would stop with Thor and clone as many of the Asgardians as possible.
And Finally, Fire Giants
The fire giants, much like the frost giants and dark elves, inhabit one of the nine worlds of Yggdrasil. In the original myth of Ragnarok, the fire giants were responsible for much of the destruction caused on Earth. The Thor movies have had the continuous theme of fighting the monsters that the Norse gods protected Earth from; I don't see why they wouldn't introduce a third race. It's even possible that the frost giants will partner with the fire giants to eradicate Asgard and move on to capture all of the nine realms.
Of course, all of this is just speculation. But I really do think that something similar to one of my theories will show up in the movie to some degree.